This invention relates to novel non-photosensitive silver dimer compounds and their use in imaging compositions, materials and methods. In particular, it relates to novel silver dimer compounds comprising two different silver salts. These asymmetric dimer compounds are useful in thermally-developable imaging materials such as thermographic and photothermographic imaging materials.
Silver-containing thermographic and photothermographic imaging materials (that is, heat-developable photographic materials) that are developed with heat and without liquid development have been known in the art for many years.
Thermography or thermal imaging is a recording process wherein images are generated by the use of thermal energy. In direct thermography, a visible image is formed by imagewise heating a recording material containing matter that changes color or optical density upon heating. Thermographic materials generally comprise a support having coated thereon: (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a hydrophilic or hydrophobic binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225xc2x0 C., resulting in the formation of an image.
Thermal recording materials become photothermographic upon incorporating a photosensitive catalyst (such as a silver halide) that upon exposure to irradiation energy (ultraviolet, visible or IR radiation) is capable of providing a latent image. This latent image can be developed by application of thermal energy. Photothermographic materials are also known as xe2x80x9cdry silverxe2x80x9d materials.
In such materials, the photosensitive catalyst is generally a photo-graphic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag0)n, also known as silver specks, clusters, nuclei, or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [D. Klosterboer, Imaging Processes and Materials (Neblette""s Eighth Edition), Sturge, Walworth and Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pp. 279-291, 1989]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide, have also been reported to be useful in place of silver halide as the photocatalyst in photothernographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishli, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].
The photosensitive silver halide may be made xe2x80x9cin situ,xe2x80x9d for example, by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in-situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Usanov et al., J. Imag. Sci. Tech. 40, 104 (1996)]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Usanov et al., International Conference on Imaging Science, Sep. 7-11, 1998)
The silver halide may also be xe2x80x9cpreformedxe2x80x9d and prepared by an xe2x80x9cex situxe2x80x9d process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to, and be present during, the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example, U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.
The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as xe2x80x9cfatty acidsxe2x80x9d or xe2x80x9cfatty carboxylic acidsxe2x80x9d. Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver containing-clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a xe2x80x9cdeveloper,xe2x80x9d may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent for silver ion. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
The imaging arts have long recognized that the field of photo-thermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50xc2x0 C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30xc2x0 C. to 50xc2x0 C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the xe2x80x9cchemistryxe2x80x9d for imaging is incorporated within the material itself. For example, they include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called xe2x80x9cinstant photographyxe2x80x9d, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of xe2x80x9cfogxe2x80x9d or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is, in the aqueous fixing step).
In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, and in Zou et al., J. Imaging Sci. Technol. 1996, 40, 94-103.
While a number of useful thermographic and photothermographic products are available in the market for medical and graphic arts uses, there is a continuing need for improving the reactivity of the compounds used to provide reducible silver ions. In particular, there is a need for imaging materials that have improved image stability and that can be imaged and/or developed at lower temperatures, while providing high Dmax, and maintaining good image tone and quality.
The present invention provides a non-photosensitive silver dimer compound comprising two different silver salts, provided that when the two different silver salts comprise straight-chain, saturated hydrocarbon groups as the silver coordinating ligands, those ligands differ by at least 6 carbon atoms.
In preferred embodiments, the non-photosensitive silver dimer compounds of this invention are represented by the following Structure I: 
wherein each E is independently oxygen, sulfur, nitrogen, selenium, or tellurium, R and Rxe2x80x2 are different alkyl, aryl, aromatic heterocyclic, or halo groups,
provided that when E is oxygen, and R and Rxe2x80x2 are both straight chain saturated hydrocarbon groups, then R and Rxe2x80x2 differ from each other by at least 6 carbon atoms.
This invention also provides a composition comprising at least first and second non-photosensitive silver dimer compounds, at least the first silver dimer compound being one of those described above for this invention.
This invention further provides a thermally-developable emulsion comprising:
a) a source of non-photosensitive silver that is a silver dimer compound as described above,
b) a reducing composition for the non-photosensitive silver dimer compound, and
c) a binder.
Further, a thermally-developable imaging material comprises a support having thereon, in the same or different thermally-developable imaging layers,
a) a source of non-photosensitive silver that is a silver dimer compound as described above,
b) a reducing composition for the non-photosensitive silver dimer compound, and
c) a binder.
A photothermographic material of this invention comprises a support having thereon, in one or more photothermographic imaging layers:
a) a photosensitive silver halide,
b) a source of non-photosensitive silver that is a silver dimer compound as described above,
c) a reducing composition for the non-photosensitive silver dimer compound, and
d) a binder.
Still again, the present invention includes a method of providing an image comprising imagewise heating the thermally-developable material of this invention.
In addition, a method of providing an image also comprises:
A) imagewise exposing the photothermographic material of this invention to imaging radiation to provide a latent image, and
B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image.
This invention also comprises a method of making the non-photosensitive silver dimer compounds of this invention, which method comprises one of the following:
A) adding a solution containing silver ions to a dispersion or a solution of a mixture of alkali metal or ammonium salts of organic ligands, or
B) adding a solution containing silver ions to a dispersion of a mixture of alkali metal or ammonium salts of fatty acids at a temperature below the Krafft temperature of the metal or ammonium salts, or
C) adding a solution of silver ion to a solution of a mixture of alkali metal or ammonium salts of fatty acids at a temperature above the Krafft temperature of the metal or ammonium salts.
The solution or dispersion may be in an aqueous or an organic solvent.
Another embodiment of this invention is a method of making a photosensitive imaging composition comprising:
A) preparing a dispersion of photosensitive silver halide grains,
B) adding to the dispersion of photosensitive silver halide grains, silver ions and first and second compounds capable of forming first and second silver salts, respectively, provided that when the first and second compounds capable of forming silver salts comprise straight-chain, saturated hydrocarbon groups as silver coordinating ligands, those ligands differ by at least 6 carbon atoms.
Thermographic and photothermographic materials incorporating the novel silver dimer compounds of this invention as the non-photosensitive silver salts can provide images with improved image stability that can be developed at lower temperatures, while providing high quality images with high Dmax and good image tone.